1. Field of the Invention
This invention relates to an improved intraocular lens adapted for implantation in a human eye. More specifically, the invention relates to a bifocal intraocular lens.
2. Description of the Prior Art
The basic concept of surgically providing a patient with an intraocularly implantable lens as a substitute for the human crystalline lens is generally known. Historically, such lenses have predominatly been made from a high molecular weight polymer such as polymethylmethacrylate. However, U.S. Pat. No. 3,996,627 proposes the use of certain selected glass compositions, while U.S. Pat. No. 4,079,470 discloses the use of low density natural and synthetic crystal such as carborundum, sapphire, ruby, zironium, strontium titanate, diamond and anatase. Various alternative methods and means of attaching the intraocular lens to the eye have been suggested and employed, including posterior chamber implantation, anterior chamber implantation as well as iris fixed intraocular lens. Several specific problems associated with the use of intraocular lens have been addressed in patent literature and elsewhere with varying degrees of success. For example, both U.S. Pat. Nos. 4,010,496 and 4,077,071 disclose the use of either an air space or attachment means of lower density to create a ballast effect or achieve neutral buoyancy. U.S. Pat. No. 4,010,496 also proposes the use of Frensnel optics with a lens demarcation line positioned such that as the pupil changes from the dilated state to the constricted state a bifocal effect is created. Unfortunately, such a structure allows for close vision only in relatively shaded light and exclusively far vision in bright light.
In U.S. Pat. No. 4,174,156 an optical lens for differential correction comprising two transparent walls defining an enclosed space having therein a quantity of transparent liquid with predetermined optical index is disclosed. In this lens, the bifocal effect is achieved by virtue of the liquid moving in and out of the line of sight in response to the tilting of the eye up and down, thus changing the power of the lens. However, as originally presented in U.S. Pat. No. 4,174,156, this lens is intended to be a contact lens or eyeglass lens and no method of achieving the desired bifocal effect within the physical confines of an intraocular lens is disclosed.